Sulfate based, ammonium free trivalent chromium decorative plating process

ABSTRACT

The present invention refers to an electroplating bath for electroplating a chromium or chromium alloy layer, the bath comprising trivalent chromium ions, organic carboxylic acid, sulfate ions, sodium conductive ions, and additives in the form of inorganic sulfur compound and boric acid as well as a process using such an electroplating bath.

The present invention refers to an electroplating bath for electroplating a chromium or chromium alloy layer, the bath comprising trivalent chromium ions, organic carboxylic acid, sulfate ions, sodium conductive ions, and additives in the form of inorganic sulfur compound and boric acid as well as a process using such an electroplating bath.

Chrome deposits from trivalent chrome electrolytes are widely used in the industry due to their unique properties they allow substrates to work longer and under tougher conditions that they would normally survive in.

During recent decades, deposition methods relying on trivalent chromium are more common for health- and environment purposes. Indeed Hexavalent chromium substances are under regulatory pressure due to their toxic nature. They were classified as CMR and the European Union decided to submit its use to specific authorization under REACH regulations.

Decorative chrome plating is designed to be aesthetically pleasing and durable. Thicknesses range from 0.05 to 0.5 μm, however they are usually between 0.13 and 0.25 μm. Decorative chrome plating is also very corrosion resistant and is often used on car parts, tools and kitchen utensils.

But the hexavalent chrome deposits were featuring a blue-white appearance that is distinguishing from the trivalent chrome deposits. This colour is still very appreciated by customers that are used to hexavalent chrome products.

JP2009035806 describes a trivalent chromium plating bath and a method for producing a chromium plating. This plating bath contains (1) complex solution of trivalent chromium obtained by maintaining under heating at least 1 type of component chosen from group which consists of aqueous solution aliphatic carboxylic acid and its salt, and aqueous solution containing trivalent chrome compound, (2) conductive salt (3) buffer for pH, and (4) at least 1 type of sulfur-containing compound chosen from the group having an SO2 group. The drawback of such a plating solution is the use of a sulfur-containing organic compound instead of inorganic one and they do not use iron in the plating bath.

JP2010189673 describes novel trivalent chromium plating bath capable of forming a trivalent chromium plating film having better corrosion resistance as compared with the prior art. A trivalent chromium plating bath comprising an aqueous solution containing a water-soluble trivalent chromium compound, a conductive salt, a pH buffer, a sulfur-containing compound and an aminocarboxylic acid. The drawback of such a plating bath is the lack of sodium and iron ions in the plating bath which will not get the desired color.

WO2019117178 describes a trivalent chromium plating solution containing: a trivalent chromium compound; a complexing agent; potassium sulfate and ammonium sulfate as conductive salts; a pH buffer; and a sulfur-containing organic compound. The trivalent chromium plating solution is practical and has a high plating deposition rate. The drawback of such a plating solution is the use of a sulfur-containing organic compound instead of inorganic one and they do not use iron in the plating bath.

EP2411567 describes a chromium electroplating solution comprising a chromium electroplating solution comprising: (1) a water soluble trivalent chromium salt; (2) at least one complexant for trivalent chromium ions; (3) a source of hydrogen ions sufficient to create a pH of from 2.8-4.2; (4) a pH buffering compound; and (5) a sulfur-containing organic compound. The chromium electroplating solution is usable in a method for producing an adherent metallic coating on a decorative article, such coating having enhanced resistance to corrosion in environments containing calcium chloride. The drawback of such a solution is the use of a sulfur-containing organic compound instead of inorganic one and the absence of iron ions in the solution.

None of those prior art documents has focused on obtaining L, a, b values close to those from hexavalent chrome deposits for trivalent chrome decorative application with a good corrosion resistance and high rate of deposition averaging 0.4 μm in 5 minutes.

When starting from this prior art it was therefore the objective of the present invention to provide a chromium plated products obtained with a good rate of deposition with a good corrosion resistance (able to pass Volkswagen test PV1073 A) with L, a, b values (comprised from 80 to 85, −0.8 to 0, −0.5 to 1.0) values close to those from hexavalent chrome deposits.

This problem is solved by the electroplating bath with the features of claim 1, the method for preparing an electroplated product by using the electroplating bath with the features of claim 10. The further dependent claims describe preferred embodiments.

According to the present invention, an electroplating bath for depositing a chromium or chromium alloy layer is provided which comprises

-   -   a) at least one source of trivalent chromium ions,     -   b) at least one source of sulfate ions,     -   c) at least one organic acid as a complexing agent,     -   d) sodium saccharin,     -   e) at least one polyalkylene glycol,     -   f) sodium vinyl sulfonate,     -   g) at least one inorganic sulfur compound,     -   h) at least one pH buffer, and, optionally,     -   i) at least one source of ferric or ferrous ions.

It was surprisingly found that the sulfate based trivalent chromium ions bath allows to obtain a whiter colour of the plating opposed to chloride based bath that get a darker plating with a higher carbon percentage. With the conductive ion, the choice of the sodium is preferred to increase the whiteness of the plating. The use of ferric or ferrous ions also increase the corrosion resistance permitting to pass the PV1073 A test. The combination of ferric, sodium and sulfate ions permit to obtain a blue and white colour close to those from hexavalent chrome deposits.

It was also found that the use of an inorganic sulfur such as an oxyacid anion containing sulfur having the valence lower than 6 is preferred. In fact, most of the time the degradation products of the organics sulfur compound cause chromability problems. The advantage of the use of oxyacid anions containing sulfur is that they will produce sulfate as a degradation product, so it will not affect the plating bath as it already contains sulfate ions. A further advantage of having an oxyacid anion containing sulfur with a valence lower than 6 in the bath is that the thicknesses of the deposits which are obtainable with the bath are higher than with baths which do not contain an oxyacid anion containing sulfur with a valence lower than 6.

It is preferred that the at least one organic acid is selected from the group of dicarboxylic acids, preferably selected from the group consisting of malic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, and mixtures thereof. In particular preferred is the use of malic acid as organic acid.

It is preferred that the concentration of the at least one organic acid is from 5 to 40 g/L, preferably from 10 to 30 g/L, more preferably from 15 to 25 g/L.

In a preferred embodiment, the concentration of the at least one trivalent chromium ion is from 5 to 25 g/L, preferably from 8 to 20 g/L.

In a preferred embodiment, the concentration of the sulfate ions from at least one source of sulfate ions is from 150 to 300 g/L, preferably from 180 to 280 g/L, more preferably from 200 to 250 g/L.

In a preferred embodiment, the source of trivalent chromium ions is chromium(III) sulphate in acidic or basic form.

It is preferred that the at least one inorganic sulfur compound is selected from the group of oxyacid anions comprising sulfur having a valence lower than 6, preferably selected from the group consisting of:

-   -   disulfite or metabisulfite,     -   dithionite or hydrosulfite,     -   thiosulfate,     -   tetrathionate,     -   sulphite and     -   mixtures thereof.

In a preferred embodiment, the concentration of the at least one inorganic sulfur compound is from 5 to 500 mg/L, preferably from 10 to 200 mg/L.

The electroplating bath can comprise at least one source of ferric or ferrous ions. The concentration of the ferric or ferrous ions from at least one source of ferric or ferrous ions is preferably from 20 to 200 mg/L, more preferably from 30 to 150 mg/L, and even more preferably from 40 to 100 mg/L.

It is preferred that the concentration of the at least one pH buffer is in a range from 50 to 120 g/L, preferably from 60 to 110 g/L, more preferably from 80 to 100 g/L.

As a pH buffer, it is preferred to use at least one of the group boric acid, citric acid, succinic acid, lactic acid, tartaric acid, and mixtures thereof. Particular preferred is the use of boric acid as pH buffer. The pH of the bath is preferably in the range from 1 to 5, more preferably from 2 to 4, and even more preferably from 3.1 to 3.9.

The concentration of the sodium vinyl sulfonate is preferably from 0.1 to 5 g/L, more preferably from 0.2 to 3 g/L.

It is preferred that the bath is (substantially) free of at least one of chloride ions, ammonium ions, amino carboxylic acid ions and hexavalent chromium ions. In particular, it is preferred that some or all of these ions are absent.

According to a preferred embodiment, the concentration of sodium saccharin is from 0.1 to 10 g/L, and more preferably from 1 to 5 g/L.

In a specific embodiment, the at least one polyalkylene glycol has a molecular weight of lower than 2000 g/mol and is preferably selected from the group consisting of:

-   -   polyethylene glycol monomethyl ether,     -   ethyleneoxide/propyleneoxyde copolymer,     -   polyethylene glycol and     -   mixtures thereof.

The advantage of having at least one polyalkylene glycol, especially at least one polyalkylene glycol with a molecular weight of lower than 2000 g/mol, in the bath is that thicknesses of the deposits which are obtainable with the bath are higher than with baths that do not contain said polyalkylene glycol.

In a preferred embodiment, the concentration of the at least one polyalkylene glycol is from 1 to 15 g/L, preferably from 5 to 10 g/L.

A preferred embodiment of the electroplating bath for depositing a chromium or chromium alloy layer comprises:

-   -   a) 5 to 25 g/L of trivalent chromium ions from at least one         source of chromium ions,     -   b) 150 to 300 g/L of sulfate ions from at least one source of         sulfate ions,     -   c) 5 to 40 g/L of at least one organic acid as a complexing         agent,     -   d) 0.1 to 10 g/L of sodium saccharin,     -   e) 1 to 15 g/L of at least one polyalkylene glycol,     -   f) 0.1 to 5 g/L of sodium vinyl sulfonate,     -   g) 5 to 500 mg/L of at least one inorganic sulfur compound,     -   h) 50 to 120 g/L of at least one pH buffer, and, optionally,     -   i) 20 to 200 mg/L of ferric or ferrous ions from at least one         source of ferric or ferrous ions.

According to the present invention, a method for preparing an electroplated product by electroplating a substrate is also provided comprising the following steps:

-   -   A) providing an electroplating bath comprising:         -   a) at least one source of trivalent chromium ions,         -   b) at least one source of sulfate ions,         -   c) at least one organic acid as a complexing agent,         -   d) sodium saccharin,         -   e) at least one polyalkylene glycol,         -   f) sodium vinyl sulfonate,         -   g) at least one inorganic sulfur compound,         -   h) at least one pH buffer, and, optionally,         -   i) at least one source of ferric or ferrous ions;

B) immersing a substrate into the electroplating bath and

C) applying an electrical current between an anode and the substrate as a cathode for depositing the chromium or chromium alloy layer on the substrate.

In a preferred embodiment, the cathode current density is in a range from 3 to 14 A/dm², preferably from 5 to 10, and/or the anode current density is in a range from 4 to 12 A/dm², preferably from 5 to 10 A/dm².

It is preferred that the anodes consist of a mixed metal oxide, preferably a mixed metal oxide selected from the group consisting of mixed metal oxides of at least two of platinum, ruthenium, iridium and tantalum, more preferably mixed metal oxides of iridium and tantalum.

In a preferred embodiment, the deposition rate during step C) is from 0.01 to 0.5 μm/min, preferably from 0.02 to 0.3 μm/min, and more preferably from 0.03 to 0.2 μm/min.

It is preferred that step C) is conducted at a temperature from 35 to 60° C., preferably from 40 to 58° C., more preferably from 45 to 55° C.

According to the present invention, the alloy obtainable from this method comprises or consists of carbon, sulphur, oxygen, chrome and, optionally, iron. The alloy has a colour measured by L, a, b values from 80 to 86, −0.8 to 0, −1.5 to 1.0. In a preferred embodiment, the L, a, b values are from 80 to 86, −0.8 to 0, −0.8 to 1. In a more preferred embodiment, the L, a, b values are from 83 to 85, −0.7 to −0.4, −0.5 to 0.2.

The percentage of carbon in the alloy is preferably from 1 to 5 atomic % (at %), more preferably from 2 to 4 at %. The alloy preferably comprises from 0.5 to 4 at %, more preferably from 1 to 3 at % sulfur. The alloy preferably comprises from 1 to 5 at %, preferably from 2 to 4 at % of oxygen. The alloy preferably comprises from 0 to 12 at % of iron. Optionally, the percentage of iron in the alloy is from 3 to 12 at %, preferably from 5 to 10 at %. The alloy preferably comprises from 74 to 94.5 at %, more preferably from 79 to 90 at %, chrome. The atomic % (at %) of the alloy can be determined by optical emission spectroscopy (OES).

With reference to the following FIGURES and examples, the subject-matter according to the present invention is intended to be explained in more detail without wishing to restrict said subject-matter to the specific embodiments shown here.

FIG. 1 shows the chromium coverage on a Hull cell panel with the three points (HCD, MCD, LCD) used for the examples.

EXAMPLES

All the examples were carried out in Hull cell (250 mL) using a brass panels nickel plated applying 5A for 5 min at 55° C. using a MMO anodes (Titanium mesh cover by mix metal oxide Ir/Ta).

The panels were evaluated: the thickness of Chromium using the X-Ray method EN ISO 3497 in three points 1 cm from the left edge define as HCD (High Current Density), 5 cm from the left edge define as MCD (Medium Current Density), 7 cm from the left edge defined as LCD (Low Current Density). The colour at the point defined as MCD was measured by a Colorimeter KONICA MINOLTA CM2600 defining the colour as CIELAB (L, a, b).

The same panels were evaluated the Chromium deposit coverage measuring the mm from the left edge to the maximum coverage of the deposit to the right. Moreover the Chromium deposit was tested to the PV1073 A that is an automotive standard used to evaluate the corrosion performance of Chromium deposit to the Calcium Chloride.

g/L Components No1 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin Sodium Hydroxide to have pH 3.5 No2 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 5 Methyl Polyethylen Glycol Mw 500 Sodium Hydroxide to have pH 3.5 No3 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 1 Sodium Vinyl sulfonate Sodium Hydroxide to have pH 3.5 No4 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 0.200 Sodium Dithionite Sodium Hydroxide to have pH 3.5 No5 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 0.050 FeII or FeIII Sodium Hydroxide to have pH 3.5 No 5 b 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharine 1 Sodium Vinyl sulfonate 0.200 Sodium Dithionite 0.050 FeII or FeIII Sodium Hydroxide to have pH 3.5 No5c 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharine 5 Methyl Polyethylen Glycol Mw 500 1 Sodium Vinyl sulfonate 0.200 Sodium Dithionite 0.050 FeII or FeIII Sodium Hydroxide to have pH 3.5 No6 115 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 5 Methyl Polyethylen Glycol Mw 500 1 Sodium Vinyl sulfonate 0.200 Sodium Dithionite 0.050 FeII or FeIII Sodium Hydroxide to have pH 3.5 No6b 55 Basic Chromium Sulphate 230 Sodium Sulphate 90 Boric Acid 25 Malic Acid 3 Sodium Saccharin 5 Methyl Polyethylen Glycol Mw 500 1 Sodium Vinyl sulfonate 0.200 Sodium Dithionite Sodium Hydroxide to have pH 3.5 No7 Reference test 250 Chromium Trioxide 1 Sulfuric Acid 1 Magnesium Hexafluorosilicate

The results of the examples are shown in the table below. The table shows how each component has a different effect in terms of thicknesses, coverage, color and performance versus PV 1073 A corrosion test.

In particular the reference example n° 7 where the deposit was carried out from Hexavalent Chromium electrolyte shows a very bluish color due to a very negative values of a and b but it didn't pass the PV1073 A test.

The present invention refers to the alloy carried out with the example n° 6 characterized in that the alloy composition contains 5-10 at % of Fe, 1-3 at % of S, 2-4 at % of C, 2-4 at % of O, remaining at % Cr (up to 100 at %) and reaching a comparable color to the reference example and a good deposition rate, with the features of claim 1 and the method for preparing an electroplated product by using the electroplating bath with the features of claim 10.

In the example n° 5 b, the bath did not contain Methyl Polyethylen Glycol (Mw 500). The disadvantage of omitting said compound in the bath is that the obtained thickness at HCD is much lower than with the bath according to the invention (bath n° 6). Besides, in the absence of said compound, the oxyacid sulphur anion (anion of sodium dithionite) (alone) is not able to increase the compliance regarding the color, the coverage and the PV 1073A.

In the example n° 5 c, the bath did not contain an oxyacid sulphur anion, i.e. did not contain sodium dithionite in the present case. The disadvantage of omitting said compound in the bath is that the thicknesses at HCD, MCD and LCD are much lower than with the bath according to the invention (bath n° 6). Color, coverage and PV 1073A are complying.

The example n° 6b shows a similar results to the n° 6 but with the better colour performance. In particular the b value reaches a very close value to the reference CrVI, wherein the efficiency is just a little bit, i.e. not significantly, reduced.

Table Showing the Results:

Thickness Color in MCD Alloy Composition by Example HCD MCD LCD L a b Coverage PV1073 A XPS profile (at %)* 1 0,12 0,1 0,05 82,8 −0,2   0,92 85/100 Corrosion 2 0,25 0,11 0,05 82,8 −0,21   0,90 86/100 Corrosion 3 0,11 0,1 0,04 83,2 −0,5 −0,1 86/100 Corrosion 4 0,18 0,4 0,18 83 −0,3   0,8 80/100 Corrosion 2-4% C 2-4% O; 1-3% S; remaining % Cr 5 0,11 0,1 0,04 82 −0,1   1,05 98/100 Unchanged 5b 0,12 0,4 0,02 83 −0,5 −0,1 95/100 Unchanged 5-10% Fe; 2-4% 2-4% O; 1-3% S 5c 0,25 0,18 0,05 83 −0,5   0,3 98/100 Unchanged 5-10% Fe; 2-4% 2-4% O; 1-3% S 6 0,35 0,4 0,02 83,5 −0,5 −0,1 95/100 Unchanged 5-10% Fe; 2-4% 2-4% O; 1-3% S 6b 0,25 0,25 0,15 84,5 −0,5 −1,0 90/100 Unchanged 2-4% C 1-4% O; 1-3% S 7 0,92 0,2 0,07 85,2 −1,1 −1,2 75/100 Corrosion 96-98% Cr; 2-4% O *Regarding the alloy composition, if not indicated otherwise, remaining % are represented by Cr. 

1-15. (canceled)
 16. An electroplating bath for depositing a chromium or chromium alloy layer, the bath comprising: a) at least one source of trivalent chromium ions, b) at least one source of sulfate ions, c) at least one organic acid as a complexing agent, d) sodium saccharin, e) at least one polyalkylene glycol, f) sodium vinyl sulfonate, g) at least one inorganic sulfur compound, h) at least one pH buffer, and, optionally, i) at least one source of ferric or ferrous ions.
 17. The bath according to claim 16, wherein the concentration of the ferric or ferrous ions is preferably from 20 to 200 mg/L, more preferably from 30 to 150 mg/L, and even more preferably from 40 to 100 mg/L.
 18. The bath according to claim 16, wherein the at least one inorganic sulfur compound is selected from the group of oxyacid anions comprising sulfur having a valence lower than 6, preferably selected from the group consisting of: disulfite or metabisulfite, dithionite or hydrosulfite, thiosulfate, tetrathionate, sulphite and mixtures thereof.
 19. The bath according to claim 16, wherein the concentration of the at least one inorganic sulfur compound is from 5 to 500 mg/L, preferably from 10 to 200 mg/L.
 20. The bath according to claim 16, wherein the at least one polyalkylene glycol has a molecular weight of lower than 2000 g/mol and is preferably selected from the group consisting of: polyethylene glycol monomethyl ether, ethyleneoxide/propyleneoxyde copolymer, polyethylene glycol and mixtures thereof.
 21. The bath according to claim 16, wherein the concentration of the at least one polyalkylene glycols is from 1 to 15 g/L, preferably from 5 to 10 g/L.
 22. The bath according to claim 16, wherein the at least one organic acid is i) selected from the group consisting of dicarboxylic acids, preferably selected from the group consisting of malic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, and mixtures thereof, preferably malic acid wherein the at least one organic acid is particularly preferred malic acid; and/or ii) comprised in a concentration from 5 to 40 g/L, preferably from 10 to 30 g/L, more preferably from 15 to 25 g/L.
 23. The bath according to claim 16, wherein the at least one pH buffer is selected from the group consisting of boric acid, wherein the pH of the bath is preferably from 1 to 5, more preferably from 2 to 4, and even more preferably from 3.1 to 3.9.
 24. The bath according to claim 16, wherein the bath is substantially free of, preferably free of at least one ion selected from the group consisting of chloride ions, ammonium ions, amino carboxylic acid ions, hexavalent chromium ions and combinations thereof.
 25. A method for preparing an electroplated product by electroplating a substrate comprising the following steps: A) providing an electroplating bath comprising: a) at least one source of trivalent chromium ions, b) at least one source of sulfate ions, c) at least one organic acid as a complexing agent, d) sodium saccharin, e) at least one polyalkylene glycol, f) sodium vinyl sulfonate, g) at least one inorganic sulfur compound, h) at least one pH buffer, and, optionally, i) at least one source of ferric or ferrous ions; B) immersing a substrate into the electroplating bath and C) applying an electrical current between an anode and the substrate as a cathode for depositing the chromium or chromium alloy layer on the substrate.
 26. The method according to claim 25, wherein the cathode current density is in a range from 3 to 14 A/dm², preferably from 5 to 10, and/or the anode current density is in a range from 4 to 12 A/dm², preferably from 5 to 10 A/dm².
 27. The method according to claim 25, wherein the at least one anode consists of a mixed metal oxide, preferably a mixed metal oxide selected from the group consisting of mixed metal oxides of at least two of platinum, ruthenium, iridium and tantalum, more preferably a mixed metal oxide of iridium and tantalum.
 28. The method according to claim 25, wherein the deposition rate during the step bis in the range from 0.01 to 0.5 μm/min, preferably from 0.02 to 0.3 μm/min, and more preferably from 0.03 to 0.2 μm/min.
 29. The method according to claim 25, wherein step C) is conducted at a temperature from 35 to 60° C., preferably from 40 to 58° C., more preferably from 45 to 55° C.
 30. An alloy obtainable by the method according to claim 25, wherein the alloy comprises from 1 to 5 at % of carbon, from 0.5 to 4 at % of sulfur, from 1 to 5 at % of oxygen, from 0 to 12 at % of iron and/or from 74 to 94.5 at % of chrome. 